1. Field of the Invention
The invention relates to a power switching technique, and more particularly, to a power switching device and a method thereof and a projection device using the power switching device.
2. Description of Related Art
A projector projects an image onto a large screen so that the image may be viewed by more people. Thus, projectors are often used in companies and schools. However, with the increasing display size of home display product, the projection device has been gradually adopted at home and become a popular electronic product in our daily life.
The design of all electronic products is going toward high speed, high performance, small volume, and light weight. As a result, the pico projector (or mini projector) has gradually become the mainstream in the market of personal digital products. Generally speaking, a pico projector adopts the light emitting diode (LED) or other solid state light sources as its light source in order to achieve an increased luminance and accordingly an increased brightness of images projected by the pico projector.
The size of a rechargeable battery in a pico projector is approximately the same as an existing mobile phone, the pico projector itself is small and light, and therefore the pico projector may be carried around conveniently. Thus, the pico projector may be used almost everywhere. In addition, a pico projector with a rechargeable battery is expected to operate for several hours. If the pico projector is used for entertainment purpose, enough power is supplied by the rechargeable battery for meeting general purposes, such as playing a DVD movie of about 150 minutes. If the pico projector is used for business purpose, it may project images from 2 to 100 inches according to the requirement of the conference report.
In an actual application, when the rechargeable battery in a pico projector has insufficient power, a user needs to use an external power adapter to keep the pico projector working. Two conventional power switching devices have been provided to safely convert the direct current (DC) power supplied by a rechargeable battery into a DC power supplied by a power supplier so as to keep a pico projector operating properly.
FIG. 1 is a diagram of a conventional power switching device 100. Referring to FIG. 1, the power switching device 100 is composed of two diodes D1 and D2. The anode of the diode D1 and the anode of the diode D2 respectively receive the DC power V1 supplied by the rechargeable battery and the DC power V2 supplied by a power supplier, and the cathode of the diode D1 and the cathode of the diode D2 are coupled with each other to output at least one of the DC power V1 and the DC power V2 as an output voltage Vout. According to the operation principle of the power switching device 100, automatic power switching is accomplished based on the voltage levels of the DC power V1 and the DC power V2. However, when the load currents on the diode D1 and the diode D2 increase, the voltage drops (i.e., cut-in voltages) of the diode D1 and the diode D2, and accordingly losses produced on the DC power V1 and the DC power V2, are also increased.
FIG. 2 is a diagram of another conventional power switching device 200, wherein the power switching device 200 is disclosed in U.S. Pat. No. 6,285,091. Referring to FIG. 2, the power switching device 200 includes two transistors (or switches) T1 and T2 and a comparator 201. The comparator 201 receives the DC power V1 supplied by the rechargeable battery and the DC power V2 supplied by the power supplier and compares the voltage levels of the DC power V1 and the DC power V2, so as to turn on at least one of the transistors T1 and T2 and output at least one of the DC power V1 and the DC power V2 as the output voltage Vout. Because little loss is produced on the load currents passing through the transistors T1 and T2, aforementioned problem in the power switching device 100 may be effectively resolved in the power switching device 200.
However, if the power switching device 200 has a great load, a great voltage drop may be produced after the DC power V1 and the DC power V2 are switched, and accordingly a system power off, hang up or reset of a portable pico projector may be resulted. If the voltage levels of the DC power V1 and the DC power V2 are close to each other, the comparator 201 may even constantly switch the transistors T1 and T2 alternatively and the output voltage Vout may be reduced. Thus, the stability of the power supply is reduced. Besides, the U.S. Pat. No. 4,788,450 also discloses a power switching technique.